Ask him to join you in:
‘Life is butter,
Life is butter,
Melon cauliflower,
Melon cauliflower…’
etc. as a round (tune: Frère Jacques). His reaction may be strongly indicative. You then hold a buttercup under his chin. This should determine guilt or innocence.

Of course you may have to fall back on that very useful Scots verdict ‘not proven’.